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Photocatalytically active TiO2/Ag2O nanotube arrays interlaced with silver nanoparticles obtained from the one-step anodic oxidation of Ti–Ag alloys Pawe# Mazierski, Anna Malankowska, Marek Kobyla#ski, Magdalena Diak, Magda Kozak, Micha# Jerzy Winiarski, Tomasz Klimczuk, Wojciech Lisowski, Grzegorz Nowaczyk, and Adriana Zaleska-Medynska ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00056 • Publication Date (Web): 09 Mar 2017 Downloaded from http://pubs.acs.org on March 10, 2017

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ACS Catalysis

Photocatalytically active TiO2/Ag2O nanotube arrays interlaced with silver nanoparticles obtained from the one-step anodic oxidation of Ti–Ag alloys Paweł Mazierskia, Anna Malankowskaa, Marek Kobylańskia, Magdalena Diaka, Magda Kozaka, Michał J. Winiarskib, Tomasz Klimczukb, Wojciech Lisowskic, Grzegorz Nowaczykd, Adriana Zaleska-Medynskaa* a

Department of Environmental Technology, Faculty of Chemistry, University of Gdansk, 80-308

Gdansk, Poland b

Department of Solid State Physics, Faculty of Applied Physics and Mathematics, Gdansk

University of Technology, 80-233 Gdansk, Poland c

Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224

Warsaw, Poland d

NanoBioMedical Center, Adam Mickiewicz University, Umultowska 85, 61-614 Poznan,

Poland

ACS Paragon Plus Environment

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ABSTRACT The development of photocatalyst with remarkable activity to degrade pollutants in aqueous and gas phase requires visible-light responded stable materials, easily organized in the form of thin layer (to exclude highly cost separation step). In this work, we presented one-step strategy to synthesize material in the form of self-organized TiO2/Ag2O nanotube array interlaced with silver nanoparticles (as in a cake with raisins) exhibited significantly enhanced photoactivity comparing to pristine TiO2 NTs under both UV and Vis irradiation. An NT array composed of a mixture of TiO2 and Ag2O and spiked with Ag nanoparticles was formed throughout the anodization of a Ti-Ag alloy in a one-step reaction. Silver NPs have been formed during the insitu generation of Ag ions and were (i) embedded in the NT walls, (ii) stuck on the external NT walls, and (iii) placed inside the NTs. The enhancement of the photocatalytic efficiency can be ascribed to the existence of an optimal content of Ag2O and Ag NPs, which are responsible for lowering the number of recombination centers. In contrast to UV-Vis light, performance improvement under Vis irradiation occurs with increasing Ag2O and Ag0 contents in the TiO2/Ag2O/Ag NTs as a result of the utilization of higher amounts of incident photons. The optimized samples reached phenol degradation rate 0.50 and 2.89 µmol·dm-3·min-1 under visible and UV light, which means an increase in degradation activity 3.8 and 2 times that of the reference samples, respectively, remained after four photodegradation cycles under UV light.

KEYWORDS: TiO2/Ag2O nanotubes, anodization, photocatalysis, Ag nanoparticles, nanocomposites.


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1. INTRODUCTION Ordered TiO2 nanotube (NT) arrays have attracted increased attention due to their high surface area, good adsorption ability, highly ordered array structure, open mesoporous nature, excellent corrosion resistance, stable physical–chemical properties and unique ability to photooxidize organic compounds

1–8

. Considering their unique properties, NTs can be used as photocatalytic

materials; however, the low visible light utilization and high recombination rate of photoexcited electron-hole pairs limit their applications 3,9–11. The photocatalytic activity of TiO2 under visible light can be achieved via loading metal nanoparticles (NPs) onto NTs hetero-coupling

15

and doping

4,16,17

12,13

. Recently, TiO2 NT arrays modified by Ag NPs and Ag2O

have been investigated in the photocatalytic degradation of acid orange methyl orange

13,19

and glyphosate

, dye sensitization 14,

13

18

, p-nitrophenol

18

,

. Silver NPs are applied to improve photocatalytic activity

because of their ability to absorb irradiation in the visible spectrum through localized surface plasmon resonance (LSPR) excitation and because of their ability to trap electrons and thereby reduce the rate of electron–hole recombination

20,21

. Recently it was stated also that plasmonic

metal NPs can efficiently convert the energy of visible photons into the energy of hot charge carriers within the nanoparticles

22

. Ag2O is also beneficial to the formation of p−n

nanoheterojunctions with TiO2 for the production of superior photocatalysts

23

. Thus, the

modification of NTs by Ag NPs and/or Ag2O should be beneficial, and this type of structure theoretically could be obtained through the anodization of TiAg alloys. In the literature, the most often presented preparation routes of Ag-TiO2 or Ag2O-TiO2 photocatalysts have been electrochemical deposition 24, photodeposition 23, precipitation method 23

, sputtering technique

25,26

, chemical reduction method

27–29

and gamma-ray radiolysis

30,31

.

Generally, Ag-TiO2 or Ag2O-TiO2 are prepared by two-step synthesis processes, which appears


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to be complicated and needed to accurately rationalize several parameters of preparation conditions. In contrast to the other methods, the electrochemical anodization approach provides a dense and well-defined layer, where NTs are perpendicularly aligned to the substrate surface. Moreover, the growth of well-defined TiO2 NTs with controllable lengths is achieved in a very short time. Recently, this method has been more and more popular as a straightforward, cost effective preparation process to produce NTs of high purity at relatively low temperature. Furthermore, it is expected that this type of method allows incorporation of silver species (Ag NPs and/or Ag2O) not only on the surface but also in the lattice of TiO2. Until today, NT-Ag2O arrays were prepared by magnetron sputtering a TiAg layer onto Ti foil, followed by anodization, and then applied as functional coatings and biomaterials

32

. TiO2 NT

arrays doped with Zr 33,34, Cr 35, Mn 36, Cu 37, Mo 38, Ni 39,40, Ta 34, Nb 34,41,42, V 43, W 44, and Fe 45

have been prepared by the one-step anodic oxidation of titanium-metal alloys. However,

information regarding the photocatalytic activity of TiO2/Ag2O NTs prepared by the anodization of Ti–Ag alloys is still lacking in the literature. The following can be expected: (i) the anodic oxidation of Ti–Ag alloys will form TiO2 NT arrays with Ag2O, (ii) TiO2/Ag2O NTs will show enhanced photocatalytic activity under UV and visible light, and (iii) the photoactivity will be affected by the amount and chemical characteristics of the silver species incorporated into the TiO2 NTs. Instead of loading of Ag into the preformed TiO2 NT arrays, one step anodic oxidation of Ti-Ag alloy in the fluoride-based electrolyte was applied to form of self-organized TiO2/Ag2O NT arrays for the first time in this work. Moreover, we demonstrate that the anodic oxidation of TiAg alloys led to the formation of TiO2/Ag2O NTs interlaced with silver NPs as in a cake with raisins. According to our best knowledge, this phenomenon has been described for the first time


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in this study. The effects of the silver content in the TiAg alloy and of the applied anodization voltage on the NT morphology and on the chemical characteristics and amount of silver species embedded into the NT walls, as well as on the photocatalytic activity of the TiO2/Ag2O/Ag NTs under UV-Vis and visible irradiation, were systematically investigated. Moreover, the formation mechanism of the TiO2/Ag2O NTs interlaced with Ag nanoparticles through anodic oxidation of Ti/Ag alloy, as well as the excitation mechanism under visible light is proposed for the first time. 2. EXPERIMENTAL SECTION 2.1. Materials Isopropanol, acetone, methanol, ethylene glycol, ammonium fluoride, phenol and terephthalic acid were purchased from POCh S.A. Titanium alloys were purchased from HMW Hauner (Germany). All aqueous solutions were prepared using deionized (DI) water with conductivity of 0.05 µS. 2.2.Preparation of TiO2/Ag2O nanotube (NT) arrays interlaced with silver nanoparticles (NPs) Prior to use, the titanium alloys were cleaned by sonication in acetone, isopropanol, methanol and deionized water for 10 min in each solvent. Then, the Ti-Ag alloys were dried in an air stream. The anodization experiments were carried out in a two-electrode electrochemical set-up. A piece of Ti-Ag alloy (3 x 2 cm) was used as the working electrode, and platinum mesh was used as the counter electrode. Additionally, a Ag/AgCl reference electrode was used to control the process and obtain information about the actual working electrode potential. Anodization was performed in an electrolyte composed of ethylene glycol, deionized water (2 vol%) and ammonium fluoride (0.09 M) for 1 h in a voltage range of 30–50 V using a programmable DC power supply (MCP M10-QS1005). To measure the actual current and potential of the Ti electrode versus Ag/AgCl, a reference electrode digital multimeter (BRYMEN BM857a) was


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applied. The as-prepared TiO2/Ag2O/Ag NTs were rinsed with deionized water followed by sonication in deionized water (5 min), drying in air at 80°C (24 h) and calcination at 450°C (in air, heating rate of 2°C/min) for 1 h. The pristine TiO2 NT arrays were prepared using Ti sheets with the same preparation conditions. 2.3.Characterization The morphology of obtained TiO2/Ag2O/Ag NT arrays was determined by using scanning electron microscopy (SEM, FEI Quanta 250 FEG). NTs lengths were measured using a tilted (30°) sample holder. The results given in the text are corrected for the effect of tilt. The SEM Jeol 7001TTLS microscope (operated at 10 kV) with Energy Dispersive X-Ray Spectroscopy (EDS) was applied to determinate actual Ag content in the samples. The morphology, distribution and location of silver in the TiO2/Ag2O/Ag NTs samples were studied using HighResolution Transmission Electron Microscopy (HRTEM Jeol ARM 200F). The UV-Vis reflectance spectra of TiO2/Ag2O/Ag NT arrays were recorded on Shimadzu UV-Vis Spectrophotometer (UV 2600) equipped with an integrating sphere. X-ray photoelectron spectroscopic (XPS) measurements were performed using the a PHI 5000 VersaProbe (ULVACPHI) spectrometer with monochromatic Al Kα radiation (hν = 1486.6 eV) from an X-ray source operating at 100 µm spot size, 25 W and 15 kV. The high-resolution (HR) XPS spectra were collected with the hemispherical analyzer at the pass energy of 23.5 eV, the energy step size of 0.1 eV and the photoelectron take off angle 45° with respect to the surface plane. The binding energy (BE) scale of all detected spectra was referenced by setting the BE of the aliphatic carbon peak (C-C) signal to 285.0 eV. Sample’s composition was checked by X-ray diffraction method using X’Pert Pro MPD Philips diffractometer, with Cu Kα radiation λ = 1.5418 Ǻ. The measurements were performed on the 2θ range of 20 to 80 degrees. The photoluminescence (PL)


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measurements were carried out at room temperature using LS-50B Luminescence Spectrophotometer equipped with Xenon discharge lamp as an excitation source and a R928 photomultiplier as detector. The excitation radiation (300 nm) was directed on the sample’s surface at an angle of 90°. 2.4. Photocatalytic measurement Photocatalytic properties of TiO2/Ag2O/Ag NT arrays were studied using two model reactions: phenol decomposition and •OH radicals generation efficiency (using terephthalic acid) under UV-Vis (λ> 350 nm) and Vis (λ> 420 nm) irradiation. Terephthalic acid reacts with hydroxyl radicals forming highly fluorescent product: 2-hydroxyterephthalicacid. Both types of photocatalytic activity tests were carried out in a photoreactor made of quartz with the working volume of about 10 mL. The NT samples with the surface area of 4 cm2 were immersed in phenol (C0 = 0.21 mM) or terephthalic acid solution (C0=0.5 mM) for 30 min in the dark to achieve adsorption-desorption equilibrium. Reference phenol and terephthalic acid samples (0.5 mL) were taken just before starting irradiation and subsequent samples (0.5 mL) were collected at regular time periods (20 min) during irradiation. Aqueous solution containing model compound was continuously stirred (500 rpm) and irradiated using a 1000 W Xenon lamp (Oriel 66021). To perform UV-Vis and Vis light induced photoreaction, the light beam was passed through GG350 and GG420 filter to cut-off wavelengths shorter than 350 and 420 nm, respectively. Irradiation intensity was measured by an optical power meter (HAMAMATSU, C9536-01) and equaled 30 and 4 mW/cm2 for UV-Vis and Vis irradiation, respectively. Phenol concentration was estimated by the colorimetric method after derivatization with p-nitroaniline using UV–Vis spectrophotometer (λmax=480 nm). Fluorescence spectra were recorded at room temperature by using a LS-50B Luminescence Spectrophotometer equipped with Xenon


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discharge lamp as an excitation source (excitation wavelength 315 nm) and a R928 photomultiplier detector. The concentration of silver in photocatalytically treated aqueous solution was determined by Atomic Absorption Spectroscopy (AAS) in an acetylene-air flame (AAnalyst 400, Perkin Elmer). The wavelength for metal was 328.07 nm (silver detection limit was 0.002 mg/dm3). For better understating of phenol degradation pathway, concentration of main degradation intermediates in the aqueous phase as well as CO2 concentration in the gas phase above reaction mixture, have been measured. Kinetics of phenol degradation by-products have been followed by high performance liquid chromatography (HPLC, Shimadzu). The HPLC system was equipped with a Kinetex C18 column (150 mm x 3 mm; particle size of 2.6 µm; pore diameter 100 Å). The flow rate was maintained at 0.4 ml/min with a mobile phase composed of acetonitrile and water (v/v: 7.5/92.5), with a sample injection volume of 30 µL, and the SPD-M20A diode array detector was operated at 205 nm. The analysis of CO2 concentration was performed using gas chromatograph (Trace 1300, Thermo Scientific) equipped with Thermal conductivity detector (TCD) and Elite-5 capillary column. The samples (200 µL) were dosed by using a gas-tight syringe. 3. RESULTS AND DISCUSSION The structural, physical and photocatalytic properties of TiO2/Ag2O/Ag NTs prepared in this study as well as preparation method are summarized in Tables 1-2. To prepare the self-organized TiO2/Ag2O/Ag NT arrays, Ti-Ag (5, 10 and 15 wt.% silver) alloys and a Pt mesh were used as the working and counter electrodes, respectively. NT growth was achieved in an ethylene glycolbased electrolyte containing 0.09 M NH4F and 2 vol.% H2O by applying a voltage in a range of 30-50 V for 60 min. For comparison, pristine TiO2 NT arrays were prepared under the same


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preparation conditions using Ti sheets. Figure 1 shows the surface and cross-sectional morphologies of the obtained pristine TiO2 and TiO2/Ag2O/Ag NTs. As observed in the SEM images in Figure 1a, highly ordered, top-end-opened and uniform NTs with an average length of 1.5-6 µm and a tube diameter of 70-120 nm were successfully obtained on all alloys and Ti sheets. The oxide ripples on the outer walls of the NTs were not observed. Notably, the silver content in the Ti-Ag alloy did not influence the dimensions of the TiO2/Ag2O/Ag NTs (Figure 1b and Table 1). Similar to pristine TiO2 NTs 6, the dimensions of the TiO2/Ag2O/Ag NTs can be easily controlled over a wide range by changing the applied voltage, for example.

Figure 1. Surface and cross-sectional morphology of pristine TiO2 and TiO2/Ag2O/Ag NTs (a), schematic view of TiO2/Ag2O/Ag NTs dimensions as the function of silver content in the Ti-Ag alloy (b) and anodization potential (c)

Figure 1c shows that the tube morphology can be increased by increasing the applied potential, starting from approximately d = 70 nm and l = 1.5 µm (30 V) and reaching approximately d =


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115 nm and l = 6.0 µm (50 V). Furthermore, the NTs obtained by the anodization of Ti-Ag alloys had the same length but a smaller diameter than the pristine TiO2 NTs, presumably because the growth rate of silver oxide is close to that of TiO2 NTs (explained further, formation mechanism of TiO2/Ag2O/Ag NTs). Table 1. Sample labels, preparation conditions and NTs dimensions (based on SEM measurements) Sample label Ti_30V Ti_40V Ti_50V Ti95Ag5_30V Ti90Ag10_30V Ti85Ag15_30V Ti90Ag10_40V Ti90Ag10_50V

Material of working electrode

Ti foil Ti(95%)/Ag(5%) alloy Ti(90%)/Ag(10%) alloy Ti(85%)/Ag(15%) alloy Ti(90%)/Ag(10%) alloy Ti(90%)/Ag(10%) alloy

External Anodization diameter voltage (V) (nm) 30 40 50 30 30 30 40 50

80 100 120 70 70 70 90 115

Tubes length (µm)

Wall thickness (nm)

1.5 3.0 6.0 1.5 1.5 1.5 3.0 6.0

10 13 18 12 12 12 15 20

To observe the distribution and location of silver in the TiO2/Ag2O/Ag NTs, Ti90Ag10 sample was chosen for TEM analysis, and the results are shown in Figures 2. Anodization of the Ti90Ag10 alloys allowed the formation of a TiO2/Ag2O NT array interlaced with Ag NPs as in a cake with raisins that were (i) embedded into the walls of the NTs, (ii) deposited on the NTs and (iii) located inside the NTs (Figure 2). Silver particles embedded into the NT walls were about 10 nm in size, whereas the Ag loaded on the NT surface were spherical and anisotropic in the range between 5 and 150 nm. No large silver particles embedded into the NT walls were observed, probably due to growth restrictions (e.g. thickness of the NT walls). In order to verify the localization of particular forms of silver nanoparticles additional studies including Energy Electron Loss Spectroscopy (EELS) and analysis of high resolution TEM images were carried out. Based on obtained result it was confirmed that nanoparticles can exist in various forms i. e. as Ag NPs, oxidized Ag NPs, and what is more as complexes consisting of Ag and TiO2. It was found that the crystalline structure of some population of particles inside


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and outside nanotubes is typical for Ag NPs with the interplanar distance dhkl(111) about 2.34Å (see Figure S1a). EELS experiments provided more accurate data about composition of investigated materials. Data presented in Figure S1b clearly shown that NPs can occur also in even more complex state within the structure which is composed of Ag and TiO2 and Ag nanoparticles which is partially oxidized.

Figure 2. Distribution and location of silver species in the Ti90Ag10 sample

Based on the experimental results presented above (SEM and TEM analyses) and literature data, a probable formation mechanism of TiO2/Ag2O/Ag NTs was proposed and is shown in Figure 3. No significant difference in the shapes of the current density-time curves registered for the anodization of the Ti90Ag10 alloy and pristine Ti sheet was observed, indicating similar formation processes. In the first stage of anodization, the current density decreased rapidly, and a TiO2Ag2O barrier oxide layer simultaneously formed on the surface of Ti-Ag alloy due to the interaction of Ti and Ag with O2− or OH− ions. During the second stage, the current density began to increase due to the local activation of the surface oxide layers and the initiation of pore


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growth. The TiO2-Ag2O barrier oxide layer is attacked by fluoride ions, leading to the formation of highly soluble (TiF6)2− and AgF products. Although there are no reports on the stable existence of (AgF2)−, AgF is soluble, with a solubility of 1.8 kg/dm3 46, and offers the possibility for a dissolution reaction. In the last stage of anodization, the current density reached quasisteady state due to the equal distributions of the available current between individual pores, and the porous structure started to convert into a nanotubular TiO2/Ag2O/Ag structure. Additionally, a small part of Ag+ could be electrochemically re-reduced to metallic silver in the form of NPs because the reduction potential of Ag+/Ag0 equals 0.7994 V. It can be concluded that the Ag2O and Ag NPs were embedded in the walls of the NTs during the growth of TiO2/Ag2O/Ag NTs. At the same time, the Ag NPs were partly exposed (deposited on and located inside the NTs) as a result of chemical etching, and Ag+ was reduced at the electrode/electrolyte interfaces.

Figure 3. Formation mechanism of the TiO2/Ag2O/Ag NTs obtained by anodization of Ti–Ag alloys


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Additionally, to confirm the presence of the Ag2O and Ag NPs in the NTs, XPS analysis of the obtained TiO2/Ag2O/Ag NTs was performed, and the results are shown in Figure S2 (Supporting Materials) and Table 2. The Ag 3d high-resolution (HR) XPS spectra, recorded on the TiO2/Ag2O/Ag samples, revealed both metallic Ag0 and Ag+ surface species represented by the Ag 3d5/2 signals at 368.3 and 367.6 eV, respectively (Figure S2)

32,47–49

. The relative

contributions of Ag+ species (Table 2) indicate that Ag2O were the main silver fraction in all samples. The Ti 2p spectrum could be resolved into two components at binding energies at 458.7 and 457.3 eV and are assigned to Ti4+ and Ti3+, respectively (Figure S2 and Table 2). Intensities of the decomposed components suggest that Ti4+ is the dominant surface state. In the case of TiO2 NTs formed from pure Ti foil, the largest contribution of the Ti3+ species was found in the Ti_40V and Ti_50V samples (2.62% of total Ti content), while the lowest contribution in the Ti_30V sample (1.52% of total content of Ti). For TiO2/Ag2O/Ag NTs, the content of Ti3+ species varied from 2.14 to 2.36%. The Ti95Ag5_30V sample contains the highest amount of reduced species in the form of Ti3+ ions, among all Ti-Ag samples. Fluoride and carbon species are common contaminants of TiO2 nanotubes obtained by anodic oxidation and both originate from electrolyte

8,50,51

. Due to high carbon content in as-prepared nanotubes grown in organic

electrolyte, Ti3+ species were induced in the pristine TiO2 and TiO2/Ag2O/Ag NTs during the calcination step 8. Furthermore, for a given electrolyte, with increasing of applied voltage the oxidation rate is enhanced which leads to an increase of generation of defective oxide at the metal/metal oxide interface. Formation of structural defects may be also controlled by the water content in the electrolyte. It was reported that the enhanced generation of Ti3+ is observed with decreasing of H2O content in the electrolyte

52

.Two XPS peaks were identified in the F 1s HR

spectra recorded on Ti90Ag10_40V and Ti85Ag15_30V (see Figure S2). The dominant peak at BE


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around 689 eV can be assigned to the incorporated fluorine ions substituting oxygen ions in the TiO2 lattice

53

, whereas the small peak located at 684.5 eV is characteristic for surface state of

fluorine adspecies 53. The amount of lattice doped fluorine in Ti90Ag10_40V sample was found to be almost two times larger than in Ti85Ag15_30Vsample, what indicates the fluorine incorporation is more effective in samples anodized at 40 V. Table 2. Surface properties and photoactivity of TiO2/Ag2O/Ag NTs and reference samples (pristine TiO2 NTs)

Sample label

XPS analysis

Average crystallite size (nm)

∑ Ti (at.%)

Ti 4+ 458.7eV (%)

Ti 3+ 457.3 eV (%)

Ag (wt.%)

33 27 36 31 38 35 36 43

36.50 50.71 52.12 49.00 47.18 43.06 45.19 44.85

98.42 97.38 97.38 97.64 97.82 97.86 97.70 97.74

1.58 2.62 2.62 2.36 2.18 2.14 2.30 2.26

0 0 0 1.25 3.93 8.37 3.70 1.62

Ti_30V Ti_40V Ti_50V Ti95Ag5_30V Ti90Ag10_30V Ti85Ag15_30V Ti90Ag10_40V Ti90Ag10_50V

Ag+ 367.6 eV (%) 88.16 72.43 84.34 93.53 84.64

Ag0 368.3 eV (%) 14.84 27.57 16.18 6.47 15.36

Photocatalytic reaction rate, r (µmol·dm-3·min-1) UV-Vis Vis light light (λ>420 nm) (λ>350 nm) 1.25 0.04 1.35 0.13 1.44 0.15 1.61 0.30 2.52 0.38 1.83 0.50 2.66 0.50 2.89 0.41

Moreover, energy dispersive X-ray spectroscopy (EDS) was applied to determinate actual Ag content in the selected photoactive layers. The average elements content was calculated based on EDS analysis in ten points of each sample. Obtained results (shown in Table 3) reflect titanium and silver content inside of photoactive layers at about 1 µm in depth. It indicates that silver was introduced along the height of NTs. The measured content of Ag in the TiO2/Ag2O/Ag NTs layer is about 3-4 times lower than that of substrate alloy and increased with increasing the Ag content in the Ti-Ag alloy. Table 3. Actual Ti and Ag weight fraction in the selected TiO2/Ag2O/Ag samples based on EDS measurements Sample label

Ti content (wt. %)

Ag content (wt. %)

Theoretical Ti/Ag ratio (weight)

Ti/Ag ratio based on EDS analysis (weight)

Ti95Ag5_30V Ti90Ag10_30V Ti85Ag15_30V

44.80 43.79 44.87

1.35 2.19 5.37

16 9 5.7

33.2 20 8.4


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Regarding the crystalline structure, all TiO2 and TiO2/Ag2O/Ag NTs were present only in the form of anatase (see Figure S3). The X-ray diffraction patterns of untreated Ti (black) and Ti95Ag5 (red) sheets are presented in panel (a) as a reference. The gradual development of the anatase surface on the Ti sheet with increasing applied potential is displayed in panels (b-d). The intensity of the XRD peaks of TiO2 increased, whereas the intensity of the peaks of Ti metal decreased. Adding silver changed the crystallographic orientation of the Ti-Ag alloy because the most intense reflection was (002), leading to a preferred orientation effect that was visible in the diffraction patterns (e-g) for samples with Ag concentrations of 5 wt.%, 10 wt.% and 15 wt.%. The latter pattern contained additional impurity peaks: Ag2O and AgTi2

54,55

, binary phases

(15 wt.% Ag is well above the Ag solubility limit in Ti) 56. The anatase crystallite size, estimated using the Scherrer equation, was approximately d = 35(7) nm and did not depend on the applied electrical potential. The evident absorption of both TiO2 and the TiO2/Ag2O/Ag NTs in the wavelength region below 380 nm was due to the intrinsic interband transition absorption of anatase TiO2, whereas the absorption in the visible light region may have been associated with trapped electrons at the Ti3+ center and with the scattering of light caused by pores or cracks in the NT layer (Figure 4a) 4,12,57

. The TiO2/Ag2O/Ag NTs had higher absorbance intensity compared to the pristine TiO2

NTs, which was consistent with the results of Zhang et al 58 and Jia et al 59. The absorption band ranging from 450 to 540 nm in the visible light region could be attributed to the LSPR effect of Ag

58,59

. The absorption peak of Ag NPs is size dependent

60

. The absorption band of Ti90Ag10

anodized at 40 V and 50 V showed a red shift compared to the Ti anodized at 40 V and 50 V. This band could indicate a small size of the Ag NPs 60. Increasing the concentration of Ag in the


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samples anodized at 30 V led to an increase in the absorption intensity as well as a red shift in the absorption edges, accompanied by increased light absorption intensities due to the introduction of narrow band-gap Ag2O 18. Compared with the typical plasmon peak of Ag NPs at 410 nm, this red shift might also be related to the interaction between Ag and TiO2 as well as the increased size of the Ag NPs 58. Photoluminescence (PL) spectroscopy was applied to characterize the electron–hole recombination behavior of the obtained samples (see Figure 4b), and these results indicated that the TiO2/Ag2O/Ag NTs were more efficient than pristine TiO2 NTs in the charge carrier separation process. As mentioned previously, Ag0 can act as traps to capture the photoexcited electrons, thus inhibiting electron recombination

20,21

. The enhanced charge carrier separation of

TiO2/Ag2O/Ag NTs is believed to also be related to the p-n heterojunction effect between p-type Ag2O and n-type TiO2 heterojunction

23

. On the other hand, the recombination rate depends on

not only the length of the NTs but also the amount of silver species incorporated into the NTs. Evidently, minor amounts of Ag species in the TiO2/Ag2O/Ag NTs significantly enhances the charge carrier separation.

Figure 4. Absorbance (a) and photoluminescence (b) spectra of pristine TiO2 and TiO2/Ag2O/Ag NTs


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The correlation between the properties and photocatalytic activities was studied using phenol decomposition (presented as the reaction rates for 60 min of irradiation) under UV-Vis (λ> 350 nm) and Vis (λ> 420 nm) irradiation, as shown in Figure 5. It should be noted, that all of the TiO2/Ag2O/Ag NTs revealed significantly higher photocatalytic activity (more than two times) compared to TiO2 NTs decorated by silver nanoparticles, obtained by anodic oxidation of Ti foil followed by radiolytic deposition of Ag nanoparticles, as presented in our previous work 5. Kinetics of photocatalytic degradation of phenol, photostability in phenol degradation reaction in subsequent cycles and photocatalytic decomposition of phenol under the influence of visible light irradiation in the presence of scavengers are presented in Figure S4. As evident from Figure 5a-d, all TiO2/Ag2O/Ag NTs exhibited significantly higher photoactivity under both UV-Vis and Vis irradiation than the pristine TiO2 NTs, which suggests a synergistic effect between the Ag2O, Ag NPs and TiO2 NTs. In order to discuss the effect of the silver content in the Ti-Ag alloy on the photoactivity of TiO2/Ag2O/Ag NTs under UV-Vis and Vis irradiation, the Ti95Ag5, Ti90Ag10 and Ti85Ag15 alloys were anodized at 30 V for 60 min, as shown in Figure 5a-b. From Figure 5a, TiO2/Ag2O/Ag NTs prepared using Ti90Ag10 had the highest phenol degradation ability (2.52 µmol·dm-3·min-1) under UV-Vis irradiation; under Vis irradiation the reaction rate increased with increasing silver content in the Ti-Ag alloy reaching 0.50 µmol·dm-3·min-1 (see Figure 5b). These results may be attributed to the different roles played by silver species under UV-Vis and Vis irradiation. Additionally, in order to consider the influence of the length of TiO2/Ag2O/Ag NTs, the Ti90Ag10 alloy was anodized at a voltage range of 30-50 V for 60 min and the results are shown in Figure 5c-d. In agreement with the asprepared pristine TiO2 NTs and the literature

61,62

, the phenol degradation rate over the


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TiO2/Ag2O/Ag NTs under UV-Vis irradiation increased linearly from 2.52 to 2.89 µmol·dm3

·min-1 with the length of tubes increasing from 1.5 to 6 µm.

Figure 5. Photocatalytic properties of pristine TiO2 and TiO2/Ag2O/Ag NTs under UV-Vis (left panel) and Vis irradiation (right panel)


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Additionally, obtained TiO2/Ag2O/Ag nanocomposite displayed photostability in phenol degradation reaction in subsequent cycles. Phenol degradation efficiency remained at level about 77% after 60 min. UV-Vis irradiation after four cycles in the presence of the Ti90Ag10_50V sample (see Figure S4e). To investigate the durability of obtained TiO2/Ag2O/Ag NTs, the Ti90Ag10_50V sample has been tested in phenol decomposition process under UV-Vis irradiation and then used sample was irradiated by high powered UV lamp (to clean the surface), and then re-use in phenol degradation reaction (this procedure has been applied four times). It was observed that the phenol degradation efficiency reached approximately 77% after four times re-use, and thus obtained TiO2/Ag2O/Ag NTs exhibited the high durability. Furthermore, the AAS analysis was applied to investigate eventual leaching of Ag+ ions from the NTs surface during the photocatalytic experiments. AAS analysis revealed no presence of Ag+ ions (< d.l.) in the phenol solution after 60-min. UV-Vis irradiation in the presence of the Ti90Ag10_50V sample. Thus, obtained results indicated that Ag+ ions have not leached out from the photocatalytic surface. From the observations of the reaction rates under visible light (Figure 5d), the 3 µm-long TiO2/Ag2O/Ag NTs had the highest photoactivity (0.5 µmol·dm-3·min-1) among the series. When the TiO2/Ag2O/Ag NTs layer is thicker than the penetration depth of visible light, the bottom part of the NTs probably absorbs only small amounts of incident photons and acts only as an intersupport. On the other hand, the mass transfer can be limited by a longer diffusion pathway, which consequently leads to a decrease in the photocatalytic degradation rate. For a more detailed comparison of the obtained results, the relationship between the reaction rate and content of Ag species (based on XPS analysis) was more suitable for revealing the differences in the photocatalytic activities under both UV-Vis and Vis irradiation, as shown in Figure 5e-f.


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Thus, under UV-Vis irradiation, the efficiency enhancement might be associated with the existence of optimal contents of Ag2O and Ag NPs (2.85 and 1.0 wt.%. respectively), which can play significant roles in the electron–hole separation process and are responsible for lowering the number of recombination centers. In contrast to UV-Vis light, Figure 5f shows that performance improvement under Vis irradiation occurs with increasing Ag2O and Ag NP contents in the TiO2/Ag2O/Ag NTs as a result of the utilization of a greater amount of incident photons. Slight visible light response of pristine TiO2 nanotubes could be related to the presence of Ti3+ and fluoride species (see data shown in Table 2). Ti3+ states may cause the appearance of impurity states located below (about 0.1-0.8 eV) conduction band, which can enhance the visible light response and creates recombination centers, preventing electron–hole recombination process

63

,

while fluorine species, which introduces localized electronic states just below the bottom of the conduction band (bulk fluorine) and can generated reduced centers (surface fluorine) 53,64, which in turn leads to improved photocatalytic properties 65,66. Additionally, to follow the intermediates formation over TiO2/Ag2O/Ag NTs (Ti90Ag10_50V sample) under both UV-Vis and Vis irradiation, high performance liquid chromatography was applied. The obtained results revealed that catechol (RT=3.2 min), hydroquinone (RT=3.7 min), benzoquinone (RT=5.4 min) and resorcinol (RT=8.1 min) were primary by-products of phenol degradation (see details in Figure 6). As evident from Figure 6a, under UV-Vis irradiation, the concentration of primary by-products reached an optimum and then decreased in parallel with the decrease of the phenol content. In contrast to UV-Vis irradiation, under Vis light, the concentration of two by-products: catechol and hydroquinone increased during the irradiation, while benzoquinone and resorcinol content started decreasing after 40 min of irradiation. The same tendency has been observed in other studies

67–70

. Moreover, lower concentration of


20

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intermediates was observed under Vis light in comparison with UV-Vis irradiation. The phenol degradation rate under Vis light was lower than under UV-Vis irradiation (see Table 2 and Figure 5) and thus, lower content of by-products was generated. Especially, the results obtained by UV-Vis irradiation suggest that portion of phenol was mineralized during the photocatalytic process. To confirm this theory, observation of CO2 formation during the UV-Vis and Vis irradiation of phenol in the presence of Ti90Ag10_50V was performed (data not shown). The presence of CO2 in the gas phase above reaction mixture was confirmed after 40 min of exposure to UV and Vis irradiation, which confirms phenol mineralization. However, UV-Vis lightinduced reaction resulted in about 4 times higher concentration of CO2 in the gas phase compared to Vis irradiation. Thus, it could be clearly stated that photocatalytic process under UV-Vis irradiation occurs faster and more efficiently.

Figure 6. Evolution of primary intermediates upon (a) UV–Vis, (b) Vis irradiation in the presence of the Ti90Ag10_50V sample; and (c) chemical structures of intermediates formed during phenol irradiation


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To determine what type of active species is responsible for the decomposition of phenol under both UV-Vis and Vis irradiation, •OH radical generation tests were performed. Figure 7a shows the correlation between the photocatalytic reaction rate and •OH radical generation efficiency under UV-Vis irradiation as a function of the silver content in the Ti-Ag alloy. A higher amount of •OH radicals were generated over pristine TiO2 NTs than in the presence of TiO2/Ag2O/Ag NTs (but which have a significantly greater ability to degrade phenol), suggesting that other forms of reactive oxygen species O2•−, HO2•, and H2O2 are more responsible for phenol degradation under UV-Vis light than •OH radicals. A similar effect was also observed under Vis irradiation, as shown in Figure 7b. To confirm the role of the generated reactive oxygen species in the photocatalytic process under Vis irradiation, tert-butanol and benzoquinone were used as scavengers of hydroxyl and O2•− radicals, respectively (see Figure S4f). Only 5% phenol degradation was observed in the presence of benzoquinone, whereas adding tert-butanol caused a negligible decrease in photocatalytic efficiency (approximately 2%). These results confirm the crucial role of O2•− in the photocatalytic degradation process. Basis on the presented results, two possible pathways can be considered to explain the photocatalytic mechanism over TiO2/Ag2O/Ag NTs under visible light, as shown in Figure 7c. The photocatalytic mechanism over TiO2/Ag2O/Ag NTs should be considered very carefully because TiO2/Ag2O/Ag photocatalysts are composed of three phases (TiO2, Ag2O and Ag0) and this mechanism seems to be complicated, as shown in Figure 7c. Firstly, for the Ag/TiO2 NTs, the mechanism of plasmon-enhanced photocatalysis is based on the local electric field enhancement associated with plasmonic NPs

71–73

. The silver NPs absorb the visible light

excitation through their LSPR phenomenon, which leads to the excitation of the energetic electrons from the Ag NPs to the conduction band of TiO2 and then to the formation of


22

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superoxide anion radicals (O2•− and then H2O2. and HO2•). At the same time, Ag2O can be excited under visible light irradiation due to the narrower band gap (approximately 1.3 eV 74).

Figure 7. (a) Correlation between the photocatalytic reaction rate and •OH radical generation efficiency under UV-Vis irradiation as a function of the silver content in the Ti-Ag alloy, (b) •OH radical generation efficiency under Vis irradiation and (c) proposed photocatalytic mechanism over the TiO2/Ag2O/Ag NTs under visible light

The photogenerated electrons are transferred to the conduction band of TiO2, where they are involved in the formation of superoxide anion radicals (O2•−) and then H2O2 and HO2•, as shown in Figure 7c. The oxidation potential of photogenerated holes of Ag2O is not high enough to


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generate •OH radicals because the electrode potential is 1.24 eV 74, lower than that for generating •

OH radicals with electrode potential of 2.80 eV. Therefore, the photogenerated holes could

participate directly in the oxidation of phenol. This also can explain the excellent stability of TiO2/Ag2O/Ag NTs because the photogenerated holes and electrons were rapidly consumed, reducing charge carrier recombination and the resultant photocatalytic activity was enhanced. On the other hand, the quick transfer of photogenerated electrons can effectively avoid the occurrence that Ag+ is reduced to Ag0 by capturing electrons accompanied with decomposition of Ag2O

18

. Thus, the simplified photocatalytic degradation mechanism of phenol presented as

the reactions was proposed as follows: TiO2/Ag2O/Ag NTs + hv (visible light) → e− +h+

(1)

e− + O2→ O2•−

(2)

O2•− + H+→ HO2•

(3)

e− + O2•− +2H+→ H2O2

(4)

h+; H2O2; O2•−; HO2• + phenol → by-products +CO2 + H2O

(5)

CONCLUSION In summary, one step anodic oxidation of Ti-Ag alloy in the fluoride-based electrolyte led to formation of novel material in the form of self-organized TiO2/Ag2O NT arrays interlaced with Ag nanoparticles, exhibiting significantly enhanced photoactivity compared to pristine TiO2 NTs under both UV and Vis irradiation. Anodization of Ti-Ag alloys allowed the formation of a TiO2/Ag2O NT array containing Ag nanoparticles in three forms: (i) embedded into the NT walls, (ii) deposited on the NTs, and (iii) located inside the NTs. Benefiting from the structure, the TiO2/Ag2O/Ag NTs not only exhibit excellent photocatalytic phenol degradation performance with rate of 2.89 and 0.50 µmol·dm-3·min-1 under UV-Vis and Vis irradiation,


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respectively, but also show stability within 77% degradation efficiency after four cycles under UV-Vis light. The enhancement of UV-induced photocatalytic efficiency can be ascribed to the existence of an optimal content of Ag2O and Ag NPs, which are responsible for lowering the number of recombination centers. In contrast to UV-Vis light, performance improvement under Vis irradiation occurs with increasing Ag2O and Ag0 contents in the TiO2/Ag2O/Ag NTs as a result of the utilization of higher amounts of incident photons due to the plasmon resonance effect and the presence of narrow band gap semiconductor (e.g. Ag2O, Eg=1.3 eV). Phenol irradiation over TiO2/Ag2O/Ag NTs led to formation of four primary by-products: catechol, hydroquinone, benzoquinone and resorcinol, and finally they were mineralized into CO2 and H2O under both UV-Vis and Vis irradiation. Furthermore, the photocatalytic activity under Vis irradiation is attributable not to •OH but to other forms of reactive oxygen species, such as O2•−, H2O2 and HO2• radicals. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: High-resolution (HR) XPS spectra, TEM jmages, X-ray diffraction patterns, Kinetics of photocatalytic degradation of phenol, photostability in phenol degradation reaction in subsequent cycles and photocatalytic decomposition of phenol under the influence of visible light irradiation in the presence of scavengers. AUTHOR INFORMATION Corresponding Author *A. Zaleska-Medynska: E-mail: [email protected] Author Contribution


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A.Z-M supervised and directed the project. A.Z-M, P.M, conceived the concept; P. M, A. M, M. K, M. K, M. D, M. J. W, T. K. W. L, G. N. performed the experimental work and analyzed the experimental data. P.M, A. Z-M participated in manuscript revision. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research was financially supported by the Polish National Science Center (research grant, Ordered TiO2/MxOy nanostructures obtained by electrochemical method; contract no. NCN 2014/15/B/ST5/00098). REFERENCES (1)

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